1. Field of the Invention
The present invention relates to a wiring board having a through hole and a production process of such a wiring board.
2. Description of the Related Art
Recently, under pursuing compaction, thinning, lightning, and multi-functionality of electronic equipment, various technical developments of a wiring board for highly densely mounting of electronic parts which constitute the electronic equipment have been intensively carried out together with compaction and thinning of the electronic parts.
Especially nowadays, it is highly desired with rapid developments of mounting techniques to provide an inexpensive multi-layer printed wiring board on which semiconductor chips such as LSI chips can be mounted highly densely and which can adapt to a high speed circuit.
In order to meet such a desire, a substrate for circuit formation is disclosed (in Japanese Patent Kokai Publication No. 6-268345), in which wiring patterns which are so laminated on an insulation substrate that they sandwich it, are electrically connected by an electrically conductive material filled in through holes or via holes.
FIGS. 9(a) to (f) schematically show steps for the production of such a substrate for the circuit formation. It is to be noted that FIG. 9 shows cross sectional views which are perpendicular to main surfaces of the substrate (also in the other drawings, the same cross sectional views are shown). The substrate for the circuit formation is produced as follows:
As shown in FIG. 9(a), through holes 2 are first formed at predetermined positions through an insulation substrate 1 made of a resin impregnated fibrous sheet 1 as a prepreg such as an aramid fiber/epoxy resin composite of which each side is coated with a non-tacky release film 17 (which may be referred to as a parting film).
Then, the through holes 2 are filled with an electrically conductive paste 3 by for example printing, followed by removing the release films 17 to have a state as shown in FIG. 9(b). The release paste comprising electrically conductive particles (for example metal fine particles) and a thermosetting resin component as a binder protrudes above each surface of the insulation substrate 1 in an amount corresponding to a thickness of the release film 17. It is to be noted that the protrusion is shown exaggeratedly for ease of understanding (the same is applicable to the other drawings).
Next, a copper foil 4 is placed on the each side of the resin impregnated fibrous sheet 1, and by pressing and heating the sheet, the resin impregnated fibrous sheet 1 in a prepreg state and the conductive paste 3 are compressed, the resin contained in the sheet and the paste is cured, and the copper foils 4 are bonded to the sheet 1 as shown in FIG. 9(c). Since the resin impregnated fibrous sheet 1 includes at least some voids therein, it is compressed when it is heated and pressed. Simultaneously, the resin of the conductive paste penetrates into the inside of the resin impregnated fibrous sheet 1 when heated and pressed, so that a concentration of the conductive fine particles in the paste is increased (that is, the conductive fine particles are dandified, and therefore an electrical resistance of the conductive fine particles is reduced, which increases reliability of the electrical connection with the paste). Thus, simultaneously with bonding the copper foils 4 to the both sides of the resin impregnated fibrous sheet 1, the copper foils 4 are electrically connected with the through holes which are filled with the conductive paste 3.
Then, the copper foils 4 on both sides of the resin impregnated fibrous sheet 1 are etched with the conventional photolithography method, and wiring patterns 5a and 5b are formed, so that a wiring board 6 having the wiring pattern on each side is produced as shown in FIG. 9(d).
Further, the produced two-sided wiring board 6 is located between other insulation substrates 1a and 1b which function as intermediate connectors as shown in FIG. 9(e). The intermediate connectors are produced by the steps of FIGS. 9(a) and (b) so that they have the through holes 2a and 2b at predetermined positions which are filled with electrically conductive materials 3a and 3b, respectively. Then, copper foils 7a and 7b are placed on the outsides of the insulation substrates 1a and 1b (namely, the sides remote from the insulation substrate 1).
Thereafter, the wiring board 6, the insulation substrates 1a and 1b, and the copper foils 7a and 7b are heated and pressed so that a multi layer integrated body is formed, of which copper foils 7a and 7b as the outermost surfaces are then etched by the conventional photolithography method. Thus, a multi layer wiring board 9 having the four wiring layers as shown in FIG. 9(f) can be obtained in which the wiring patterns 5a and 5b on the resin impregnated fibrous substrate 1 are electrically connected with the wiring patterns 8a and 8b by the conductive paste 3a and 3b filled in the through holes 2a and 2b. 
With the resin impregnated fibrous sheet such as an aramid fiber/epoxy resin prepreg in which a non-woven fabric of aramid fibers is impregnated with an epoxy resin, the aramid fibers contained therein cause irregularities on surfaces of the resin impregnated fibrous sheet, and for example, the resin impregnated fibrous sheet 1 has surfaces having irregularities which directly correspond to forms of the aramid fibers.
When the wiring patterns 5a and 5b are formed by etching the copper foils 4 bonded with heat and pressure to such a resin impregnated fibrous sheet 1, the copper foils 4 also have irregularities on their surfaces due to the irregularities of the resin impregnated fibrous sheet 1, which is likely to cause voids or gaps between the copper foils 4 and etching resist layers (not shown). Upon etching, an etching solution penetrates into the gaps, so that it becomes difficult to form wiring pattern 5a and 5b as predetermined. Especially, a problem is caused in that it is difficult to form fine wiring patterns, which accommodate the highly densely mounting of microelectronic parts such as semiconductor bare chips.
Further, the resin impregnated fibrous sheet is in such a state that a resin material is impregnated into a substrate made of the fibers, so that a structure of the resin impregnated fibrous sheet is unlikely to be uniform as a whole. For example, for the resin impregnated fibrous sheet such as an aramid fiber/epoxy resin prepreg in which the non-woven fabric of the aramid fibers is impregnated with an epoxy resin, since the aramid fibers are present discontinuously or exposed partly on surface skin layers of the sheet, the following two problems are likely to occur:
1) When the resin impregnated fibrous sheet is bonded with heat and pressure to a copper foil, the resin impregnated in the resin impregnated fibrous sheet contributes to the bond between them. An amount of the resin is small in portions under the copper foil where the aramid fibers are present or exposed partly on the surface skin layer of the insulation substrate. As a result of this, since the bond between the copper foil and the resin impregnated fibrous sheet is insufficient, no sufficient bonding strength is available between them. Thus, when electronic parts are mounted on a wiring pattern on such a wiring board, no large mounting strength is available; and
2) When the surfaces of the resin impregnated fibrous sheet have the irregularities, adhesion strength between the release film 17 and the resin impregnated fibrous sheet 1 is insufficient during the production steps of the wiring board as shown in FIGS. 9(a) to (f), so that the conductive paste 3 is likely to penetrate into gaps between the release film 17 and the resin impregnated fibrous sheet 1 upon filling the conductive paste 3 into the through holes. As a result of this, the conductive paste 3 extends beyond a diameter of the through hole 2 over the insulation substrate 1. Thus, when a separation between the through hole and the wiring pattern formed on the substrate, or between the through hole and other through hole is small, an electric short circuit is likely to be formed between them. This limits the formation of highly dense wiring patterns.
On the other hand, there has been proposed a method of forming a fine wiring pattern in which the fine wiring pattern is beforehand formed on a release supporting plate separately, and the fine wiring pattern is transferred to a surface of an insulation substrate (see Naoki FUKUTOMI et al., xe2x80x9cDevelopment of Fine Line Printed Wiring Technology by Plated Wiring Pattern Transfer Methodxe2x80x9d, IEICE ""89/4 vol. J72-C No. 4, pp 243-253 (1989)). However, even when such a method is employed, since the surface of the resin impregnated fibrous sheet as the insulation substrate has the irregularities, it is difficult to precisely transfer the fine wiring pattern onto the insulation substrate.
Also, as a method which improves the adhesion strength between an insulation substrate and a metal foil such as a copper, there is disclosed a method in which a surface of a resin impregnated fibrous sheet as the insulation substrate in which an epoxy resin is impregnated is coated with an adhesive resin (see Japanese Patent Kokai Publication No.8-316598). In this method, the impregnated resin of the sheet is in contact with the adhesive resin which is compatible to the impregnated resin while both of them are uncured, and then they are heated and pressed so that they are compressed and cured and thus bonded together. Thus, it is difficult to fully avoid the effects of the irregularities of the non-woven fabric surface.
Further, as a wiring board technique which makes highly dense mounting possible, there are disclosed methods in which thermosetting resin layers are provided on surfaces of both sides of an insulation substrate of a resin impregnated fibrous sheet (see Japanese Patent Kokai Publication Nos.7-263828 and 8-78803). In those methods, uncured thermosetting resin layers are formed on the insulation substrate of which resin was cured, then through holes are formed, which are filled with a conductive paste, and then the substrate is heated and pressed. Since the insulation substrate itself was already cured, the substrate is not substantially compressed, so that the conductive paste in the through holes is also not compressed, resulting in that no densification of the conductive paste is achieved in the through holes. Therefore, electrical connection reliabilities between wirings by means of the conductive paste in the through holes (resistance lowering of the conductive paste, resistance stability of the conductive paste, and ensuring adhesions between the paste and the insulation substrate and between the paste and a conductor for wiring) become insufficient.
It is an object of the present invention to overcome the above problems. That is, it is an object to provide a printed wiring board which makes it possible to mount parts more highly densely and which has an improved reliability by overcoming the above problems while keeping advantages of a substrate for circuit formation in which a conductive material filled into a through hole(s) electrically connects wiring patterns provided on both sides of an insulation substrate, and also to provide a process for the production of such a wiring board.
The inventors have made intensive studies, and completed the present inventions by finding that when a composite (which comprises a compressible resin impregnated fibrous sheet as a prepreg and a flat heat resistant film, preferably a heat resistant resin film which is compatible to the resin impregnated fibrous sheet and which is provided on at least one of main surfaces of the resin impregnated fibrous sheet) is used as an insulation substrate and a wiring pattern is formed on the insulation substrate, a surface of the insulation substrate on which the wiring pattern is formed becomes substantially flat even though a surface of the resin impregnated sheet is not flat (namely having irregularities), so that formation of a fine wiring pattern becomes possible and thereby reliability of the electrical connections of wirings is easily ensured.
That is, the present invention provides a both side or two-sided wiring board in which predetermined wiring patterns, especially fine wiring patterns formed on both sides of an insulation substrate which comprises a resin impregnated fibrous sheet and a heat resistant film provided on at least one side, preferably on both sides of the resin impregnated fibrous sheet, are electrically connected by means of an electrically conductive material filled in a through hole(s) which is formed through the insulation substrate.
In the present specification; the term xe2x80x9cboth side wiring boardxe2x80x9d is intended to mean a wiring board in which an insulation substrate (which is usually in the form of a sheet) has a wiring pattern on both sides. Also, a term xe2x80x9cmulti layer wiring boardxe2x80x9d which will be explained below is intended to mean a wiring board containing at least three wiring pattern layers which are separated from one another by insulation substrates each located between adjacent two wiring patterns, and which are connected with one another by means of an electrically conductive material filled in through holes which are formed through the insulation substrates (thus, having a structure in which the insulation substrates and the wiring patterns are located alternately).
The both side wiring board as described above is produced by a process of the present invention comprising the steps of:
(1) providing the heat resistant film on at least one side of the resin impregnated fibrous sheet so as to obtain a composite as an insulation substrate;
(2) providing a release film on an exposed surface of the heat resistant film so as to form a pre-wiring board;
(3) forming at least one through hole which passes through the insulation substrate and the release film;
(4) filling the through hole with an electrically conductive material;
(5) removing both release films from the pre-wiring board so as to obtain the insulation substrate in which the heat resistant film is exposed on at least one side of the insulation substrate; and
(6) providing a wiring material (for example a metal foil such as a copper foil) on each side of the insulation substrate, and heating and pressing the insulation substrate and the wiring materials together so as to bond them integrally, followed by forming the wiring materials into predetermined wiring patterns.
Forming the wiring materials into the predetermined wiring patterns, namely patterning may be carried out by etching the wiring materials using the conventional photolithography method.
According to the present invention, since the surface of the insulation substrate on which the wiring material is located is flat, adhesion between a photo mask and the wiring material is improved during the etching step of the wiring material using the photolithography method so that a fine wiring pattern is able to be formed precisely. As the wiring material, instead of the metal foil, an electrically conductive paste, an electrically conductive ink and so on may be used. In this case, providing the conductive material may be carried out not before, but after heating and pressing is carried out. Alternatively, when the predetermined wiring patterns are directly formed by printing, the patterning may be omitted.
According to the process of the present invention as described above, the novel printed wiring board can be inexpensively produced which includes a further improved reliabilities relative to the conventional printed wiring board having the via holes and on which fine wiring patterns are formed so that micro electronic parts can be highly densely mounted on the fine wiring patterns.
In the above production process according to the present invention, step (6) may be replaced with step (7) which entails placing onto the exposed heat resistant film, a wiring pattern supporting plate comprising a release supporting plate on which a predetermined wiring pattern has been beforehand formed while aligning with the through holes, and heating and pressing the plate and the insulation substrate together followed by removing the release supporting plate, that is, the step of transferring the wiring pattern on the plate to the insulation substrate. Generally, the release supporting plate may be made of a sheet of an electrically conductive material (for example, a stainless steel). The wiring pattern supporting plate may be produced by plating a wiring material (for example, copper) on the electrically conductive material, which is etched so as to leave the predetermined wiring pattern. For the detailed information as to the wiring pattern supporting plate, the above referenced literature xe2x80x9cDevelopment of Fine Line Printed Wiring Technology by Plated Wiring Pattern Transfer Methodxe2x80x9d can be cited, and the disclosure of the literature is incorporated by reference in this specification.
According to the present invention, since the exposed surface of the insulation substrate is substantially flat, a finer wiring pattern can be formed precisely when the transferring step is combined.
The formation of the through holes in the process according to the present invention is known also from the three patent publications which are referred to above. For example, a laser process may be used for the formation of the through holes. For the detailed information as to the through hole formation, the above patent publications can be cited, and the technical disclosures of these publications may be incorporated in this specification by reference. With the formation of the via holes as described, thin through holes which are adapted to the fine wiring pattern may be formed rapidly and easily through the insulation substrate having the release films.
It is noted that there is no specific limitation as to the conductive material which is filled into the through holes, provided that the material can be filled into the through holes, and will electrically connect wiring patterns placed on both sides of the insulation substrate thereafter. In particular, an electrically conductive paste or fine metal powder (such as fine solder powder), having good fluidity may be used. When such a conductive material is used, sufficient filling properties and highly reliable connections are ensured even though a diameter of the through hole is reduced with the wiring patterns being finer. Filling of such a conductive material in the through holes may be carried out by any appropriate method. For example, the printing method can fill the conductive paste into substantially the entirety of the through holes.